Baca insisted his decision to step down was "based on the highest of concern for the future of the Sheriff's Department. He repeatedly cited the upcoming campaign, which he said had already brought "negative perception" to the department.

"I don't see myself as the future," he said. "I see myself as part of the past."

Baca declined to endorse a candidate in the upcoming election but said he had recommended that the L.A. County Board of Supervisors appoint Asst. Sheriff Terri McDonald to oversee the department after he leaves.

He said his greatest accomplishment would be reducing crime rates, and he commended his deputies for their work.

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"They have conducted themselves with the utmost integrity and professionalism resulting in yet another year of historic crime reductions in nearly half a century," he said. "In my opinion, your Sheriff's Department is the greatest law enforcement agency in the world."

The news of Baca's decision to step down stunned people inside and outside the agency. He was locked in a tough reelection battle amid several scandals that had beset the department.

Baca told top officials in county government late Monday that he believed stepping down would help the department recover after several years of tumult and criticism, according to sources familiar with the conversations.

His announcement comes a month after federal prosecutors filed criminal charges against 18 current and former sheriff's deputies accused of beating jail inmates and visitors, trying to obstruct the FBI and other crimes following an investigation of corruption inside the nation's largest jail system.

Baca won office in 1998 after his rival, incumbent Sheriff Sherman Block, died days before the election. In the next three elections, Baca easily won in primaries against fields of lesser-known candidates, avoiding head-to-head runoff elections. By 2010, no one bothered to challenge him.

During his career Baca advocated education and rehabilitation programs inside the county jails and reached out to the Muslim community after the Sept. 11, 2001, terrorist attacks. But his tenure was also marked by periods of violence in the jails as well as overcrowding, which prompted the department to release inmates after serving only a fraction of their terms.

Recently, Baca was coping not just with the FBI probe but searing criticism of his leadership from a blue-ribbon commission appointed by the Board of Supervisors to examine allegations of jail abuse.

Last year, the U.S. Department of Justice accused sheriff's deputies of engaging in widespread unlawful searches of homes, improper detentions and unreasonable force as Antelope Valley authorities conducted a systematic effort to discriminate against African Americans who received low-income subsidized housing.

The two outside investigations portrayed a troubled department sharply at odds with the vision Baca preached during his 15 years as sheriff. More recently, The Times also reported that the department had hired dozens of officers in 2010 despite background investigations that found they had committed significant misconduct.

Doubts were growing that the previously popular sheriff would be able to win a fifth term as sheriff while facing a challenge from his former top aide, Paul Tanaka, and retired Cmdr. Bob Olmsted.

Earlier Monday, Baca had thrown his support behind a proposal to set up an oversight commission for the Sheriff's Department.

Supervisors Mark Ridley-Thomas and Gloria Molina proposed setting up a permanent civilian oversight commission in September, after the U.S. Department of Justice announced that its civil rights division would investigate the treatment of mentally ill jail inmates in county custody.

Baca publicly took a stance on the proposal in a statement declaring that such a commission would be "consistent with my view on strengthening transparency and accountability, and would serve to further develop law enforcement skills regarding constitutional policing, procedural justice, civil rights and human rights as a whole."